This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Appln. No. 100 01 516.6 filed in Germany on Jan. 15, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a nondestructive method for determining the thickness of a metallic protective layer on a metallic base material.
It is known for highly stressed turbine blades, for example guide vanes and rotor blades, or combustion-chamber parts of a gas turbine to be provided with a protective layer, in order to obtain optimum protection for the gas-turbine part against mechanical and thermal effects or against oxidation while the turbine is operating. Turbine blades which consist of a nickel-base superalloy are in widespread use in the prior art, although alloys based on a different metal are equally conceivable. An example of a known metallic protective layer is MCrAlY. M stands for the metals Cr, Ni or Fe. Various coating processes for applying a protective layer of MCrAlY are known from the prior art. For example, plasma spraying processes are described in U.S. Pat. No. 4,152,223 or U.S. Pat. No. 4,246,326, although an electrical or chemical vapor deposition process is also possible. Other equivalent processes are also conceivable. However, before commissioning and at regular maintenance work carried out on a coating installation, it is necessary to set process parameters for these application processes so that a certain protective layer thickness on the gas-turbine component can be ensured. For this purpose, in the prior art it is known for the layer thickness of an applied protective layer, during the test phase of a coating process and for monitoring purposes during general operation, to be determined by destruction of the gas-turbine component, i.e. for the gas-turbine component to be cut open and analyzed in the laboratory (chemical etching, etc.). Hitherto, the similarity between the metallic base material and the metallic protective layer meant that it was impossible to use nondestructive measurement methods. Laboratory work is in general terms very time-consuming and also labor-intensive, which in itself represents a first drawback, and secondly the gas-turbine component has to be cut open and can then no longer be used, which represents a further drawback.
The invention seeks to remedy these problems. An object of the invention is to provide a nondestructive method for measuring the thickness of a metallic protective layer on a metallic base material. This should advantageously be a gas-turbine component which can be used further after the measurement and does not have to be discarded.
According to the invention, the object is achieved by the fact that the thickness of the metallic protective layer is determined by detection of a different type of layer which is situated between the metallic protective layer and the metallic base material.
In one embodiment, the intermediate layer may advantageously be a nonmetallic layer, i.e., for example, oxide particles or carbides, which can be produced simply by sandblasting. In a further embodiment, the intermediate layer is a metallic layer, for example of pure nickel. It is also possible to create a layer with depleted Al and Ti content in that region of the base material which is close to the surface before the metallic protective layer is applied. Both these embodiments have the advantage that the gas-turbine component can be used further following the determination, since during operation the nickel diffuses into the base material of the component and the protective layer or the elements Al and Ti are replaced again by diffusion from base material and protective layer. This may also take place in the course of the downstream diffusion heat treatment. To improve detectability, it is also possible for the base material to be additionally irradiated.
Advantageously, with the present invention it is possible to use methods which are known from the prior art, such as ultrasound, (pulsed) thermography or an eddy-current technique for measuring the layer thickness, since the layer located between the two metallic materials provides a sufficiently great contrast for a measurement to take place, thus allowing nondestructive measurement.